Radio receiving system



Nov. 8, 1932-.

VOl rs APPL 150 ro DECTECTOIP.

J. M. MILLER RADIO RECEIVING SYSTEM Filed July 25. 1930 2 Sheets-Sheet lmuf 70cc 12cc 14cc Nov. 8, 1932. J. M. MILLER RADIO RECEIVING SYSTEMFiled July 25. 1950 2 Sheets-Sheet 2 Patented Nov. 8, 1932 STATS PATENTOFFICE JOHN M. MILLER, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TOATWATER KENT MANUFACTURING COMPANY, OF PHILADELPHIA, PENNSYLVANIA, ACORPORATION OF PENNSYLVANIA RADIO RECEIVING SYSTEM Application filedJuly 23,

My invention relates to radio receiving systems or sets in which beatsare produced by interaction of locally produced oscillations with theoscillations representing the received signals; and more particularly tosuperheterodyne receiving systems or sets, for reception of broadcastspeech and music.

In accordance with my invention, the local oscillator of the receiver,comprising a Vacuum tube having regeneratively magnetically coupledoutput and input circuits, is given such characteristics that thevoltage impressed upon or in advance of the input circuit of a detectoris substantially constant or varies within suitably narrow limitsthroughout a substantial range of frequencies of the oscillationslocally generated.

In accordance with my invention, there is imparted to the localoscillator system of the receiving apparatus such characteristics thatthroughout the range of frequencies of the locally produced oscillationsthe oscillator system continues near the threshold of nonoscillation,yielding oscillations which are feeble or of suitably low amplitude,reducing undesirable radiation or re-radiation, and whose elfect, asrepresented by the voltage impressed upon or in advance of the inputcircuit of the detector, is of substantially constant magnitude orvaries within narrow limits throughout the aforesaid range offrequencies.

In accordance with my invention, the oscillator system is such that theamplitude of he oscillations produced, or the magnitude of the voltageimpressed upon or in advance of the input circuit of the detector, isdependent substantially solely upon th radio or high frequencyresistance of the tunable loop circuit of the input system of theoscillator, and the input resistance or impedance remains, throughoutthe range of frequencies of the oscillations generated, of suchmagnitude, or varies within such narrow limits, that the voltageimpressed up on or in advance 1930. Serial No. 470,016.

7 of the locally generated oscillations.

In accordance with my invention, there is V bridged across the controlgrid and cathode of the oscillator tube a condenser, preferably of fixedcapacity, which is in series in a loop with an inductance and acondenser, the latter variable as to its capacity for varying thefrequency of the locally produced oscillations. The system has suchcharacteristics that the frequency of the locally generated oscillationsis closely dependent solely upon the capacity of the variable condenser,which makes practical a uni-control receiver or set in which theadjustable element of oscillator variable condenser is mechanicallycoupled for operation in unison with the adjustable element or elementsof one or more variable condensers utilized in tuning a circuit orcascaded circuits, in advance of the detector, to the frequency of thesignal representing oscillations.

More particularly, the aforesaid fixed condenser, in series with thevariable condenser and inductance, has the important function ofrendering the oscillator frequency substantially independent ofvariations in tube characteristics and their effects, which varymaterially, even though the tubes be of the same manufacture or type;substantially independent of variations in line or plate voltage or infilament current; and, in general, substantially independent of thoseeflects which have material influence upon the oscillator frequency andthe beat frequency when the input electrodes are connected to the alsoto prevent mtermittent cessation of genterminals of the commonly usedloop comprising an inductance shunted by a variable con oscillator tubea resistance so positioned and of such magnitude as to render moreconstant throughout the range of frequenciesof the locally generatedoscillations the input resistance or impedance of the oscillator tube;and more particularly, such resistance serves eration of oscillations,particularly adjacent the low frequency end of the frequency range.

My invention resides in a system of the character hereinafter describedand claimed. For an understanding of my invention, and for anillustration of some of the various embodiments thereof, reference is tobe had to accompanying drawings, in which:

Fig. 1 1s a d1agrammat1c lllustration of the commonly used localoscillator system.

Fig. 2 comprises curves or graphs illustrating the characteristics of asystem generally illustrated by Fig. 1 and of my improved system.

Fig. 3 is a diagrammatic view of a local oscillator system havingcharacteristics utilizable in accordance with my invention in radioreception of the character herein described.

Fig. 4 is a diagrammatic view of a portion of a superheterodyne radioreceiving system,

in accordance with my invention.

Referring to Fig. 1, O is a thermionic vacuum tube having control gridg, anode or plate a, and cathode 7, which, in this instance,

if is a filament maintained at suitable temperature by current from thesource 1. grid or input circuit is an inductance L1, shunted by thevariable condenser C, forming a tunable loop whose terminals areconnected to the grid and cathode. plate circuit is an inductance L2,coupled tov In the anode or L1, with the usual source 2 of plate circuitcurrent supply. This system comprises magnetically coupled input andout-put systems,

causing regeneration to the point of production of oscillations whosefrequency is to substantial extent, but not substantially entirely,dependent upon the capacity of variable condenser C, though thefrequency is dependent upon other factors, including variations involtage of the source 2, and 1n characteristics of the tube 0 and theireffects. Coupled to the inductance L1 is' an inductance In the 7 or coilL3 across whose terminals appears a voltage which may be impressed uponthe input circuit of the detector of a heterodyne or superheterodynesystem. That voltage, across the terminals of the coil L3, increasesrapidly as the condenser C is adjusted to smaller and smaller capacitiesto produce oscillations of higher and higher frequencies.

The curve A, Fig. 2, in a general way represents the voltage-frequencycharacteristic of such an oscillator. The voltage impressed upon thedetector in such case rises very rapidly with increase in frequency, andthis is undesirable, particularly in superheterodyne reception.

For the purposes of my invention, I utilize an oscillator system of thecharacter illustrated in Fig. 3, which is a modification of Fig. 1 inthat there is bridged across the grid and cathode a condenser C1,preferably of fixed capacity. The input terminals of the tube 0 are nolonger bridged across the terminals of the condenser C, but areconnected respectively to one terminal of the tunable loop, as before,and to a point 3 between the condenser 01 and the variable condenser C,which in effect is in series with C1 across the terminals of theinductance L1. A system of this character has improved characteristics,as compared with Fig. 1, for utilization in heterodyne orsuperheterodyne reception, as will presently appear. In addition thereis preferably utilized a resistance R1 shunted across grid and cathodewith beneficial effects later described.

A system, such as illustrated in Fig. 3, is utilized, in accordance withmy invention, in radio receiving sets in which a beat frequency isproduced, and particularly in superheterodyne sets for receivingbroadcast signals including music and speech. The oscillator is requiredto produce oscillations of frequencies varying from say 680 to 1630kilocycles, to produce a constant beat or intermediate frequency of,say, 130 kiloeycles, for the present broadcast frequencies ranging from550 to 1500 kilocycles. lVhile my invention is not limited to theseparticular frequencies, either of the broadcast range or of the localoscillator, they serve as an illustration of an application of myinvention.

Referring to Fig. 2, the voltage across the terminals of the coil L3varies with frequency generally in accord with the characteristic curve13, having the portions B1 and B2. The part B1 shows that the voltageapplied to the detector is substantially constant throughout aconsiderable portion of the frequency range of the oscillator. PortionB2 shows that the system has a characteristic opposite to that indicatedby the curve A, illustrative of Fig. 1, in that the voltage impressedupon the detector actually decreases with further increase of frequency,or through the high frequency portion of the range. The characteristic Bis decidedly advantageous as compared with the characteristic, such ascurve A, particularly in broadcast reception by the superhetrodynemethod where it is desirable that the voltage impressed upon the firstdetector of a superhetrodyne system shall be constant or vary withinsuitably small limits, which is the case represented by thecharacteristic B. WVith commonly used oscillators, such as indicated byFig. 1, the voltage impressed upon the detector may increase in someinstances as much as twenty times between the lowest and highestfrequencies of the entire range. Characteristic A represents a 'moremoderate change of voltage of the order of five times. variation ofabout 2, and it would be much less except for the effects which causethe downwardly curving portion B2, which is en tirely due to theoscillator system or circuit itself, but generally is caused by otherinfluences or effects due to practical considerations in the productionof superheterodyne receiving sets.

Figure t represents a portion of a superheterodyne receiving system, inaccordance with myinvention, in which there is applied a localoscillator system of the character shown in Fig. 3. f

In Fig. 4, D represents an antenna or other suitable absorptionstructure. In this case between the antennaD and earth E or ground,

or equivalent counter capacity, or the metal chassis of a receivlng set,is included the primary p to difierent taps on which connects theantenna switch I) to suit the receiving system to antennas of differentcharacteristics or lengths. Coupled to the primary p is the secondary s,in circuit with which is the primary p1. In shunt to s and p1 is thetuning condenser C2 for tuning the circuit to the frequency of thesignal oscillation. Coupled to the primary p1 is the secondary 81, inseries with which is an inductance 82, both shunted 1 or bridged by thecondenser C3 variable to tune this cascaded circuit to the frequency ofthe signal oscillations. In shunt to the inductance s2 is a condenser K.

V is the first detector, of a superheterodyne system, having a controlgrid 9, cathode c,

maintained at suitable temperature by the electric heater h, and theplate or anode a, and shield or screen grid (Z. The control grid isconnected to a point between the inductances 31 and 82, while thecathode is connected through the coil or secondary L3 and grid biasingresistance R2, shunt-ed by condenser K1, to ground E, or other terminalof the loop tuned by the condenser C3.

To the output circuit of the detector V is coupled the input circuit ofan intermediate frequency amplifier tube, with which may be cascaded oneor more similar intermediate frequency amplifier tubes, the platecircuit of the last of which is coupled to the input Characteristic Bshows a of audio frequency amplification. The intermediate frequencyamplifiers, second detector, etc., are not illustrated, since thatarrangement is per se well known.

The local oscillator O in this instance is provided with a cathode a,heated by the electric heater h. It will be understood, however, thatthe oscillator tube may have as its cathode a filament as in Fig. 3. Theparts of the oscillator are generally the same with the action andcharacteristics of a system such as illustrated by Fig. 3. The platecircuit is coupled by coil L2 to the coil L1 of the input circuit, underconditions producing oscillations. The coil L3 in the detector inputcircuit is coupled with the inductance L1, to impress upon the detectorV a voltage of radio frequency differing to fixed'extent from the signalfrequencies to which the condensers C2 and C3 tune their respectivecircuits. As indicated by the broken'line U, the rotors or adjustableelements of the three condensers C1, C2 and C3 are mechanically coupledin unison, so that as the cascaded circuits are tuned to the signalfrequency, the condenser C is varied as to its capacity in such amountas to cause the frequency of the oscillations produced by generator 0always to differ to fixed extent from the signal frequency, therebyyielding a constant beat or intermediate frequency.

In the example illustrated, for the power for heating the cathodes andsupplying current tothe plate circuits of the various tubes of the set,there is provided a transformer T whose primary is connected across thecommercial alternating current lighting or power lines 4 and 5. Thesecondary S has its terminals connected to the anodes a of the rectifiertube V1 whose filament or cathode f receives its current from thesecondary S1. The rectified current is passed through a filter systemcomprising a suitable arrangement of inductances and capacities,yielding at the terminals 6 and 7 a rectified filtered current deliveredto the plate circuits of the tubes 0, V, and of the other tubes of thesystem. Between the positive terminal 6 and the earth E, or terminal 7may exist a difference of potential of 160 v., which may be the maximumutilized on the plate circuit of any of the tubes in the system. In theplate circuit of the oscillator O, in series with the coil L2, is aresistance R3 shunted by condenser K2, of suitable magnitude, say 10,000ohms, which keeps the plate current of the tube from rising to highmagnitude, or regulates the plate'current. The resistance R3 is of suchmagnitude that the effective voltage impressed upon the plate circuit ofthe tube O'is of the order," of, say 100 Volts,

thetube 0 being of the type now commonly known as UY227 or C327.

The secondary S2 of the transformer T supplies current for the heatersh, of the severaltubes, for maintaining their cathodes at suitabletemperature. In lieu of cathodes c, filaments such as f, Fig. 3, may beutilized,

- Q in which case they receive their current from the secondary S2, or asecondarysimilar or in addition thereto.

A local oscillator having the desirable characteristics for the purposesherein described and claimed, may comprise, with tube of the typeaforesaid, coils L1, L2 and L3,

,having inductanc'es respectively, of 190, 13 and 3.5 microhenries.tance between the coils L1 and L2 is of the order of -32'microhenries,while the mutual inductance between coils L1 and L3 may be of the orderof 6 microhenries. For producing oscillations of frequencies within therange of, say 680 to 1630 kilocycles, the capacity of the variablecondenser C will vary between and 400 micro-microfarads, the

condenser C1 having a capacity of 1200 micro-microfarads, and theresistor R1 having a resistance of about 50,000 ohms.

The capacity of the condenser C1, very high in comparison with thegrid-cathode,

capacity .of the tube 0, is considerably in excess of the maximumcapacity of the variable condenser C. v

When the loop circuit L1, C, C1 is ad usted by variation of condenser Cfor any frequency within the frequency range of the locally genefatedoscillations, of the total potential difference, the portion across thea condenser C1 will be roughly inversely proportional to the ratio ofthe capacity of condenser C1 to the then capacity of the condenser C.The difference of potential across the terminals of condenser G1, whichis the potential effective across the grid and cathode of the tube 0, isa maximum for the lowest frequency of the range, and decreases to aminimum for the highest frequency. The voltage impressed upon the inputelec- "trodes of the oscillator 0, therefore, varies The mutual induc-,

across the terminals of the variable condenser C, a rel ation upon whichvery largely de- ,pends the fact that the voltage impressed upon thedetector by an oscillator, such as shown in Fig. 1, rapidly increaseswith the frequency of the locally generated oscillations.

With an oscillator system of the character of F ig. 3, oscillations aregenerated when MGm R01 is equal to or greater than unity, where M is themutual inductance between the grid and plate coils L1 and'L2, Gm is themutual conductance of the tube 0, C1 is the capacity of the condenserC1, and R is the high or vradio frequency resistance effectively inseries in the resonant loop L1, C, C1. The ratio of is so chosen that,at the highest frequency of the range, the system will produceoscillations. At the highest frequency the magnitude of R is a maximum,decreasing'with decreasing frequency. The ratio of is in a practicalcase such as illustrated in Fig. 4, so chosen that oscillations will beproduced at the highest frequency notwithstanding usual or reasonablevariations in voltage of thesupply line 4, 5. V a

The magnitude of-the oscillatory current,

v or the amplitude of the, oscillations produced, U decreases withincreasing frequency, offsetting the effect of the inductive couplingL1, L3 which produces a voltage across the terminals of the secondary L3which increases with the frequency of the oscillations, even if theiramplitude remain-constant. 'In consequence, as illustrated by thesubstantially straight portion B1, of the characteristic B, Fig. 2, thedecrease of amplitude of the oscillations with increasing frequencyoffsets the opposite characteristic of the inductive coupling to thedetector.

The resistance factor B, above referred to, increases with increasingfrequency, and in the high frequency portion of the range the rateofincrease in resistance with increase of frequency is so high that theamplitude of the oscillations produced decreases at such high rate thatnotwithstanding the aforesaid characteristic of the inductive couplingto the detector, the v voltage impressed upon the detector may, asindicated by theportion B2 of the characteristic curve B, materiallydecrease with increasing frequency. The condition, represented by theportion B2 of the characteristic, is ascribed to the cumulative effectswith increasing frequency of departure from 90 degrees difference inphase between the currents in the grid and plate coils L1. and L2, ofapproach metal masses adjacent the oscillatory system or kindred causes.

In an oscillator system of this character, with a chosen mutualinductance between grid and plate coils, only the resistance varies withfrequency, and for production of oscillations of highest frequency therelations are such that their amplitude is small, and throughout therange to the lowest frequency, the amplitude increases though in generalsmall, with a high magnitude of input impedance. With such relativelyfeeble oscillations there arises the advantage that the radiation orre-radiation from the receiving system is small, with a minimum ofdisturbing effect uponneighboring circuits or receiving systems.

In general, the oscillator throughout the frequency range operatesrelatively closely to the threshold of non-oscillation.

It is a further characteristic that the variation of frequency of theoscillations generated is dependent to unusualy high degree solely uponthe magnitude of the capacity of the variable condenser C, assumingother circuit constants fixed. The frequency is not materially affectedby the characteristics of the oscillator tube 0, as they may varybetween different specimens of the same type. The frequency is notsubstantially affected by variations in the grid-cathode capacity of thetube, nor is the frequency substantially affected by changes in voltageof the supply source, as circuit 4, 5, which ordinarily causeconsiderable changes .in frequency by resultant yariations in platevoltage and cathode temperature. I r

The resistance R1, in shunt to the condenser Gl, serves as a leakagepath from the grid of the oscillator tube. It preferably has relativelylow magnitude as of the order of 50,000 ohms, making the input impedanceof the system more uniform throughout the frequency range. In addition,it prevents intermittent interruption in the generation of oscillations,particularly at the low frequency end of the range.

Where the variation of frequency of the oscillations generated is notdependent substantially solely upon the variation of. the capacity ofthe condenser C, Fig. 1 for example, the coupling of the rotor oradjustable element of that condenser in a uni-control with theadjustable elements of the tuning condensers such as C2 and C8, Fig. 4,causes variation in the beat or intermediate fre quency ofthe receivingsystem, which in the case of a tuned intermediate frequency amplifierisundesirable.

By utilizing in a heterodyne or superheterodyne system, however, in myinvention, a local oscillator system of the character described, inwhich the variation of frequency of the generated oscillations isdependent practically solely upon the capac ity of the variablecondenser G, there obtains a substantially fixed difference between thefrequencies of the signal and locally gener-. ated oscillations withsubstantially fixed beat or intermediate frequency throughout the: rangeof frequencies of the received signals to which the receiving system istunable. In particular, in such uni-control, the frequency of the localoscillator'is substantially independent of variations in characteristicsof the vacuum tubeemployed, and independent of changes in voltage of thesupply source.

It is of great practical importance, in a, superheterodyne system,asherein described, that the local oscillatorsystem have thecharacteristics hereinbefore described, and inparticular thatthevariation of frequency of the locally generated oscillations shall bedependent. substantially entirely upon or strictly relatedto thecapacity of the variable condenser C. It is particularly desirable thatthe oscillator have this characteristic when utilized in asuperheterodyne system pro vided with highlyselective systems, one inadvance of the first detector and tunable to the desiredsignalfrequencies, and the other following the first detector and highlyselective of the beat frequency. For thispurpose it is of importance andhighly desirable, par.- ticularly in a uni-control system, such asdescribed, that the frequency of the oscillator always differs tosubstantially strictly the same extent throughout the range of desiredsignal frequencies, to yield a substantially strictly constant beatfrequency to co-act with and have full advantage of the highly selectiveintermediate frequency amplifier system. I I- claim:

1. In a heterodyne receiving system, a detector, means for impressingthereon signal representing oscillations, a local oscillator comprisinga vacuum tube having an output system coupled to its input system,inductance and a plurality of condensers, one of fixed capacity, all inseries with each other in a loop in said input system, connections fromthe terminals of said condenser of fixed capacity to the inputelectrodes of said tube, means for varying the capacityof another ofaccordance with said condensers to vary the frequency of the out thefrequencyrange of the locally pro duced oscillations. a r a 2. In aheterodyne receivlng system, a detector, means for impressing thereonsignalrepresenting oscillations, a local oscillator comprising a vacuumtube having an output system coupled to its input system, lnductance anda plurality of condensers, one of fixed'capacity, all in series witheach other in a loop in said input system, connections from theterminals of said condenser of fixed capacity to the input electrodes ofsaid tube, means or varying the capacity of another of said condensersto vary the frequency of the locally generated oscillations, means forim pressing upon said detector a voltage produced by the locallygenerated oscillations and varying within narrow limits throughtance anda plurality of condensers, one of' fixed capacity, all in series witheach other in a loop in said input system, connect-ions from theterminals of said condenser of fixed capacity to the input electrodes ofsaid tube, means for varying the capacity of another of said-condensersfor varying'the frequency of the generated oscillations, thecharacteristic of the oscillator being such that the oscillationsproduced are throughout the frequency range of the generatedoscillations of low amplitude decreasing with increase of frequency, andan inductive coupling through which the locally produced oscillationsimpress upon said detector a voltage which varieswithin narrow limitsthroughout the fretions. a

4. In a heterodyne receiving system, a 'detector, means for impressingthereon signalrepresenting oscillations, a local oscillator quency rangeof the locally generated oscilla comprising a Vacuum tube having in itsinput system a loop comprising an inductance and a plurality ofcondensers, all in series, one of said condensers variable to vary thefre: quency of the locally generated oscillations, another of saidcondensers having a capacity large withrespect to that of said variablecondenser, connections from the terminals of said last named condenserto the input electrodes of said tube, an inductance in the output systemof said tube coupled to said inductance, and means for impressing uponthe detector a voltage produced by the locally generated oscillations.

5. In a heterodyne receiving system, a vacuum tube detector, means forimpressing thereon signal-representing oscillations, a local oscillatorcomprising a vacuum tube having an output system coupled to its inputsystem, a loop in said input system compris: ing an inductance andaplurality of condensers, allin series with each other, connections fromthe terminals of one of said condensers to theinput electrodes of saidoscillator tube,

a commercial power system supplying cathode-heating and plate currentsfor said detector and oscillator tubes, an inductive coupling betweensaid inductance and the input of said detector tube, and means forVarying the capacity of another of said condensers for varying thefrequency of-the locally generated oscillations, said one of saidcondensers having a capacity large with respect to selector system fortuning to signal-repre senting oscillations varying in frequencythroughout a range, said selector system comprising at least onevariable tuningcondenser, a local oscillator comprising a vacuum tube, aloop in the input system thereof comprising inductance anda plurality ofcondensers all in series, one of said condensers having a capacity,large with respect to the variable capacity of another of saidcondensers, connections from the terminals of said one of saidcondensers to the input electrodes of said tube, the local oscillatorsystem having such characteristic'that the frequency of theoscillationsgenerated is varied substan: tially solely by the capacityof said variable condenser, an inductive coupling through which thelocally generated oscillations impress a voltage upon said firstdetector, means for mechanically coupling the adjustable ele ments ofsaidftuningcondenser and of said variable condenser of the oscillatorsystem, the characteristic of the oscillator system being such that thevoltage impressed upon said first detector varies within, narrow limitsthroughout the range of frequency of the locally generated oscillationsand ,that

throughout the frequency range of signalrepresenting oscillations thelocally generated oscillations have a frequency causing saidintermediate frequency to be constant.

7 In a superheterodyne receiving system, a first detector, a selectorsystem in advance of said detector tunable to signal-representingoscillations, said selector system comprising at least one variabletuning condenser, a second detector, a highly selective intermediatefrequency amplifier between said detectors, a local oscillatorcomprising a vacuum tube, a loop in the input circuit thereof comprisinginductance and a plurality of condensers, all in series, one or" saidcondensers having a capacity large with respect to the variable capacityof anoth r of said condensers, connections from the terminals of saidone of said condensers to the input electrodes of said tubes, :1coupling through which the locally generated oscillations impress avoltage upon the first detector, and means for mechanically coupling theadjustable elements of said tunin condenser and said variable oscillatorcondenser for movement in unison to effect selection of signals inadvance of the first detector and impression of the signals at beatfrequency upon the second condenser, the characteristic of theoscillator system being such that, as the frequency of the generatedoscillations is determined substantially solely by the capacity of theVariable condenser of the oscillator system, the beat frequency isconstant and within the range of the selective intermediate frequencyamplifier.

8. In a superhet-erodyne receiving system, a first detector, a selectorsystem in advance of said detector tunable to signal-representingoscillations, said selector system comprising at least one variabletuning condenser, a second detector, a highly selective intermediatefrequency amplifier between said detectors, a local oscillatorcomprising a vacuum tube and a variable condenser for varying thefrequency of the generated oscillations, means mechanically coupling theadjustable elements of the condensers or" said selector system and localoscillator for movement in unison, and means for insuring that the beatfrequency is within the narrow range of said selective system comprisinga loop in the input circuit of said oscillator including an inductancein series with said second variable condenser and a condenser ofrelatively large capacity connected to input electrodes of saidoscillator tube.

9. In a heterodyne receiving system comprising a detector tube and anoscillator tube for generating local oscillations and having inassociation with a pair of its electrodes a resonance loop or"inductance and variable capacity, the method of ensuring that thevoltage impressed upon the detector tube by the locally generatedoscillations is substantially independent of adjustments of saidvariable capacity, which comprises impressing upon said pair ofelectrodes of the oscillator tube a potential difference whose magnitude is dependent upon the potential difterence across a fixedcapacity in series with said variable capacity within said resonanceloop, and impressing the locally generated oscillations through aninductive coupling upon the detector.

10. In a superheterodyne receiving system comprising a first detector, atuned intermediat frequency amplifier system, a preselector system, inadvance of the first detector, includin g at least one adjustable tuningdevice, and an oscillator tube having in association with a pair of itselectrodes a resonance loop or" inductance and a condenser variable inunison with said tuning device of the preselector system, the method ofinsuring, notwithstanding variations in oscillator tube characteristicsor changes in voltage of the source supplying current to the oscillatortube, that simultaneous adjustment of said tuning device and saidcondenser shall efi'ect selection of desired signals and localgeneration of oscillations to produce beats of the frequency to whichsaid intermediate frequency amplifier system is tuned, which comprisesimpressing upon said pair of electrodes of the oscillator tube adifference of potential dependent upon the dilierence of potentialacross a fixed capacity in series within said loop with said variablecondenser.

JOHN M. MILLER.

